I found a thread discussing similar behaviour
http://serverfault.com/questions/684...y-printers-why
There is not enough information presented to determine the cause of the re-write, but it is important to stop the CUPS server first if you are manually editing /etc/printers.conf, since AFAIU CUPS keeps a live copy while running, and dynamically updates the file (eg printer status) while running, so perhaps your edit is then getting overwritten.
Anyway, it might be useful to put CUPS logging into debug mode
Code:
cupsctl LogLevel=debug
and examine /var/log/cups/error_log for references to 'Saving printers.conf...' etc.